The invention relates to a process for producing mica-containing insulating tape for winding elements of rotating high voltage electrical machinery which after winding under a vacuum and pressure can be impregnated with a solvent-free synthetic resin and then cured under the action of heat, and their use in the main insulation of rotating high voltage electrical machines.
To insulate winding elements in rotating high voltage electrical machines the impregnation technique based on vacuum pressure impregnation has become very popular in recent years. In doing so the winding elements for construction engineering reasons are made either as preformed coils or conductor bars, preferably Roebel bars. These winding elements are provided with mica-containing main insulation and are further treated in a vacuum pressure impregnation process. Here epoxy resins, preferably low-viscosity solvent-free resin systems, are used as the impregnation resins.
At this point they impregnate in a vacuum and under pressure the mica-containing main insulation formed from several winding layers so that cavities which can cause partial discharges between the winding layers are completely filled so that after curing of the impregnation resin an electrically and mechanically stable insulating sleeve is formed.
The winding layers of the main insulation are formed by mica-containing insulating tapes. They are produced such that a mica film is cemented to a porous carrier material using a binder. This porous mica-containing insulating tape is designed to absorb the aforementioned impregnation resin under a vacuum and pressure to the greatest possible extent.
Mainly in generator construction in recent years has the demand grown for mica-containing insulating tapes which are to have not only good workability on modern winding machines, but also satisfactory impregnation capacity since due to the increasing size and power of generators likewise high insulating wall thicknesses have become necessary.
Known insulating tapes can only partially meet this requirement since they act rather as barriers due to their physical and chemical composition relative to the penetrating impregnation resin so that at longer impregnation times the impregnation depth, i.e. the number of impregnated winding layers, is insufficient for high insulating wall thicknesses.
The object of this invention is therefore to devise a mica-containing insulating tape of the initially mentioned type which in addition to very good winding properties on modern automatic winding machines also exhibits much improved impregnation behavior during vacuum pressure impregnation. In addition, this mica-containing insulating tape should be easy to produce and stable in storage.
Therefore, by the invention it is proposed that this mica-containing insulating tape be produced such that a fiber-containing mica film which in addition to fine mica also has organic and/or inorganic fibers is sprinkled with an epoxy resin mixture in the form of a powder enamel and that the side of the fiber-containing mica film which has been sprinkled with the powder enamel is cemented under pressure and at an elevated temperature to a carrier material.
Preferably this carrier material consists of a glass fabric, a fabric or a plastic film.
Introducing fibers into the mica film makes the latter more porous so that its capillary action is increased relative to the impregnation resin during vacuum pressure impregnation. The resulting advantages are shorter impregnation times and impregnation with more highly viscous resins to be able to impregnate also very high insulating wall thicknesses entirely with impregnation resin.
Furthermore, preferably a fiber content is chosen which is between at least 3 and a maximum 50% by weight relative to 100% by weight fiber-containing mica film. A proportion of fiber less than 3% by weight does not contribute to improvement of the impregnation capacity; a proportion of fiber exceeding 50% by weight even reduces the dielectric strength of the mica-containing insulating tape.
Other advantages of the invention consist in that the organic and/or inorganic fibers in the fiber-confining mica film have an average diameter from 0.1 to 20 microns and an average length from 0.5 to 10 mm.
Furthermore, according to the invention the organic fibers are chosen from the group of aromatic polyamides or aromatic polyesters such as polyethylene terephthalate and the inorganic fibers are chosen from the group of E-glass, S-glass, silicon glass and aluminum.
According to another version of the process as claimed in the invention the fiber-confining mica film has a curing accelerator, preferably zinc naphthenate, which acts on the solvent-free synthetic resin.
In addition, the powdered epoxy resin mixture can contain a curing agent, preferably an amine curing agent.
According to the process of the invention the epoxy resin mixture which is used in the form of a powder enamel has a softening point exceeding 85xc2x0 C.
The invention furthermore relates to use of a mica-containing insulating tape which was produced according to the aforementioned process versions as the main insulation in rotating high voltage electrical machines, the mica-containing insulating tape being wound in several layers around a winding element and then impregnated under a vacuum and pressure with a solvent-free synthetic resin.
Furthermore, in the aforementioned vacuum pressure impregnation preferably solvent-free synthetic resins such as for example epoxy acid anhydride mixtures based on bisphenol-A or bisphenol-F are used.
Epoxy resin acid anhydride systems are usually heated to 60-70xc2x0 C. to have viscosity low enough for the impregnation process. Mica-containing insulating tapes which have been produced according to the process as claimed in the invention can however be satisfactorily impregnated at a much higher resin viscosity and are therefore especially well suited for impregnation with resins which can only be heated to roughly 30xc2x0 C. and therefore have a mostly very high impregnation viscosity, for example polyester resins with styrene or vinyl alcohol as the monomer.